The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI) techniques utilizing late echo pulse echo planar imaging (EPI) and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other magnetic resonance applications.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B.sub.0, is set up in an examination region in which a subject being imaged or examined is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped to excite magnetic resonance. The resonance is typically manipulated to induce detectable magnetic resonance echoes from a selected region of the subject. In imaging, the echoes are spatially encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix, commonly known as k-space. By employing Inverse Fourier, Two-dimensional Fourier, or other known transformations, an image representation of the subject is reconstructed from the k-space data.
Echo planar imaging (EPI) is a rapid MRI technique which is used to produce magnetic resonance echos at high acquisition rates. It has been found useful in perfusion and/or diffusion studies, for functional magnetic resonance imaging (fMRI), in dynamic-contrast studies, etc. However, images obtained in EPI experiments tend to be vulnerable to an artifact known as "N/2" or "Nyquist" ghosting that produces ghost images typically positioned at N/2 pixels relative to the true or desired object image position (where N is the number of pixels across the image field of view (FOV)). More specifically, alternating errors or cyclic errors can be generated in the k-space data due to common system limitations or imperfections such as, e.g., imperfect gradient application, non-linear system responses (i.e., Maxwell fields, mechanical displacements or vibrations, etc.), instabilities in digital to analog conversion timing, or inherent properties of the imaged object (i.e., susceptibility differences, flow/respiratory changes, chemical shifts, etc.). The cyclic errors are typically created by differences in the odd and even horizontal data lines of k-space, e.g., misalignment of the data line peaks, or phase shift errors. These may be denoted as cyclic errors, because each full cycle of the readout gradient contains both a positive polarity portion and a negative polarity portion, and within the full cycle there is typically mismatch or error between the two polarity portions. The same error is largely repeated in each successive cycle. Likewise, these errors may be denoted as alternating, because each cycle typically produces a pair of consecutive data lines, with the odd numbered lines exhibiting substantially consistent data, the even numbered lines also being substantially consistent, but the neighboring even and odd lines exhibiting relative error or inconsistency. In any event, the Fourier reconstruction tends to convert the cyclic errors into secondary images or "ghosts" that are shifted by a half-image from the primary or true desired image of the object.
The ghost images can obscure the true desired image, reduce image clarity or sharpness, and generally degrade overall image quality. Moreover, high levels of ghosting can produce false readings that lead to diagnostic error. Accordingly, it is highly desirable to produce EPI images that are essentially free of ghost artifacts.
A number of techniques have been developed for addressing ghost artifacts. However, such techniques remain subject to certain drawbacks or limitations. For example, one popular and well-known method for EPI ghost reduction employs a reference scan with zero phase encoding prior to the imaging pulse sequence. By examining offsets in the echo between even and odd echo acquisitions, a set of phase correction values is determined. The goal of the phase reference scan technique has typically been to remove zero and first order phase differences between odd and even echoes, and has been shown capable of reducing an amount of ghosting. Still, the phase reference scan technique is known to occasionally increase the N/2 ghosting artifact. Reference scan methods may introduce error into images if there is deviation or inconsistency between the reference acquisition and the associated image acquisition, or if there are flawed results generated from the analysis of the reference image.
Another method in the prior art involves the collection of EPI raw data in which a data line or small number of data lines are replicated. Time shifts and perhaps phase shifts can then be estimated by looking at the location and phases of maximal signal in each line of the free induction decay (FID) readout. However, this technique is disadvantageous insomuch as the additional data lines disrupt the continuous readout in the phase encode direction and introduces point spread errors for signals not on resonance. Additionally, estimating phase differences between alternating data lines with only two data lines can result in an inability to discern the alternating part of the signal variation from gradual linear (non-alternating) drifts which cause peak misalignment.
Moreover, many previously developed techniques are relatively complex and time intensive. For example, post-processing of the collected k-space data using ghost removal algorithms is a commonly used technique for reducing the impact of N/2 ghosting. However, such ghost removal algorithms or routines are generally computationally intensive and thus are frequently performed at the expense of additional scans and lengthen reconstruction times. Furthermore, in many cases post-processing ghost removal techniques do not adequately eliminate ghosting artifacts and, on some occasions, have been known to increase the amount of ghosting artifact. Additionally, some techniques require active operator intervention and/or judgment to effect ghost reduction, thereby putting demands on the operator's time and leaving open the possibility of operator error.
In prior art systems, the number of read encoding steps (N.sub.ro) is typically equal to or greater than the number of phase encoding steps (N.sub.pe) Additional read steps are obtained by incrementing the data sampling rate. Additional phase encode steps are obtained from additional echoes, which normally increases data acquisition times. Also, matrix sizes are often not selectable to match the geometry of the object to be imaged, but rather, are some fixed value, e.g., 64.times.64, 64.times.128, 128.times.128, 64.times.192, and so forth (wherein the first number is the number of phase encoding steps and the second number is the number of read encoding steps). For fixed sequences, the adjustments for anatomy can be made during a pilot imaging procedure. A pilot procedure is a quick series of pulse sequences to give an operator images which can be used to size the FOV. However, the adjustments for anatomy are limited and do not necessarily match the geometry of the particular geometric body as it may change from person to person of from object to object. Thus, prior art systems using fixed matrix/resolution EPI sizes produce an inefficient use of sampling time, i.e., wasting valuable k-space territory thereby resulting in inefficient resolution selection, lower image quality (e.g., due to increased partial voluming effects), and/or poor ability for reformatting images.
In addition, the prior art EPI and other multi-echo techniques lack anti-aliasing protection in the phase encoding direction, with ghost-removal routines generally being relied upon to reduce the impact of N/2 ghosting as described above.
The present invention provides a method and apparatus which overcomes the above-referenced problems and others.